Reading Disability (RD) and Attention-Deficit/Hyperactivity Disorder (ADHD) are the two most prevalent disorders of childhood, and these two disorders frequently co-occur. The overall goal of the proposed research is to refine our understanding of the diagnostic validity, etiology and neuropsychology of ADHD in order to understand how ADHD contributes to RD. To accomplish these objectives, an extensive psychometric test battery will be administered to a sample of 50 pairs of identical twins and 50 pairs of fraternal twins in which at least one member of each pair is reading disabled, to an independent sample of 50 pairs of identical twins and 50 pairs of fraternal twins in which at least one member of each pair has ADHD, to a comparison sample of 50 pairs of identical twins and 50 pairs of fraternal twins with no school history of reading deficits or ADHD, and to the siblings of all twin pairs. The test battery includes multiple measures of executive functions, orbital frontal cortex functioning, and processing speed. A well-validated diagnostic interview and a battery of rating scales will also be administered to parents and teachers of each twin and their siblings to assess ADHD and comorbid internalizing and externalizing symptoms. DNA will be collected from each twin and sibling and all biological parents, and these phenotypic and genotypic data will be used to conduct univariate and bivariate twin, linkage, and candidate gene analyses to identify quantitative trait loci that influence ADHD and its comorbidity with RD.